A conventional fiber-optic gyroscope usually comprises a light source, a first coupler, a polarizer, a second coupler, a fiber coil, and a photodetector. The light source includes a light emitting device and a monitoring photodetector. The prior gyroscope guides light beams emitted from the light emitting device into an optical fiber. The beams pass the first coupler, are polarized into linear polarization by the polarizer, and are divided into halves by the second coupler. The divided light beams enter both ends of the fiber coil and propagate clockwise (CW) and counterclockwise (CCW) in the fiber coil. Then the beams are integrated by the second coupler, are polarized again and are led to the photodetector which is furnished at an end of the fiber path. The CCW beam and CW beam interfere with each other at the photodetector. The power of the interfering beams is sensed by the final photodetector. The photodetector is positioned at a symmetric spot to the light emitting device with regard to the first coupler. The light source module has a light emitting device and a monitoring photodiode mounted behind the light emitting device. The monitoring photodiode aims at detecting the light power of the light emitting device for stabilizing the light power. A gyroscope has two photodetectors in general., i.e. a main one for detecting the object signal of the interference beams and an auxiliary one for stabilizing the light power of the light emitting device. To distinguish two photodetectors, the main one is now called a main photodetector or a main photodiode, and the auxiliary one is called a monitoring photodetector or a monitoring photodiode.
The reason why two couplers are inserted in the optical path is to equalize exactly the path of the CW beam to the path of the CCW beam. One coupler is still enough to divide light beams into halves, lead the beams to both ends of the fiber coil, integrate the beams, and send the beams to the main photodetector. However, in this case, the paths of the CW beam and the CCW beam are different. The difference of the optical paths brings about an offset between the CW beam and the CCW beam. Thus two couplers are indispensable to avoid the occurrence of the offset.
The purpose of interpositioning the polarizer in the optical fiber path is equalizing the polarizations of the CCW beam and the CW beam. Two beams with different polarizations cannot interfere with each other at all. Interference demands the same polarization of two beams. A gyroscope having a light emitting device, a main photodetector, a fiber coil, two couplers and a polarizer has been called a "minimum configuration", since these elements have been deemed indispensable. The requirement that a gyroscope must have a photodetector belonged to common knowledge.
However the Inventors of the present invention have objected to the common sense. The Inventors have proposed a gyroscope acquiring a signal from the light source instead of the final main photodetector by Japanese Patent Application No.5-57756. FIG. 2 shows the basic structure of the gyroscope suggested first by the Inventors.
A light source (1) has a light emitting device (8) and a monitoring photodiode (9). The light emitting device (8) emits monochromatic light or quasi-monochromatic light. The light beams are converged by a lens (10) onto an end of a single-mode optical fiber (2). The light beams pass a polarizer (3) and are converted into linearly polarized beams. Further the light beams are divided into halves by a coupler (4). These divided two beams are guided into ends of a fiber coil (5) which is produced by winding a single-mode fiber a plurality of turns around a bobbin. The beams propagate in the coil (5) clockwise and counterclockwise. A phase-modulator (6) is interposed at an end of the fiber coil for modulating the phase of light passing therethrough. A depolarizer (7) depolarizes the light beams passing there through. The proposed gyroscope has only a single coupler. The gyroscope has no photodetector which is used to be furnished at an end of the fiber path symmetric to the light source with regard to the coupler. Thus the new gyroscope succeeded in omitting a photodetector and a coupler.
Being emanated from the light source, the beams are divided in half by the coupler, propagate in the coil (5) counterclockwise and clockwise and return to the light source (1). The light source (1) includes a light emitting device (8) and a monitoring photodiode (9). Here the light source (1) includes an assembly of the light emitting device (8) and the photodetector (9) for monitoring the power of the light emitting device. The returning light perturbs the oscillation of a laser diode or superluminescent diode. The emission performance of the laser or superluminescent diode is greatly changed by the returning light. Then the power of the interfering light is detected from the change of the driving current of the light emitting device (laser diode or superluminescent diode) at a constant applied voltage, or the change of the applied voltage of the light emitting device at a constant current. Otherwise the power of the interfering beams can be detected from the photocurrent of the monitoring photodiode (9). Then there are three difference ways (voltage, current or photocurrent) for detecting the signal including Sagnac's phase shift. Synchronous detection of the signal (voltage, current or photocurrent) with a carrier of the same frequency as the phase modulation reveals the Sagnac phase shift.
The newly proposed source-sensitive gyroscope enjoys an advantage of being built with still fewer parts than the minimum configuration. A coupler and a photodetector have been removed in the new gyroscope. Saving of a coupler considerably reduces the cost of production, because it is rather difficult to manufacture a coupler. Omission of a photodetector can further reduces the cost of parts.
The fiber coil can be built either with a single-mode fiber (SM-fiber) or with a polarization maintaining fiber (PM-fiber) in the source-sensitive gyroscope.
The gyroscope with a coil of a polarization maintaining fiber has a benefit of maintaining the polarization of the light propagating in the coil. Further the gyroscope can do without a depolarizer, since the light beams spread in the coil with the same polarization. Nevertheless, the PM fiber is still an expensive fiber due to the difficulty of production. Adoption of the polarization maintaining fiber enhances the cost of production of the gyroscope.
The gyroscope with a coil built by a single-mode fiber enjoys the advantage of low cost of production. The single-mode fiber gyroscope, however requires further contrivance. A single-mode fiber is unable to keep the polarization of light in a certain direction due to the rotational symmetry. Accidental rotation of polarization induces the fluctuation of the scale factor and the drift of the zero-point. Thus the single-mode fiber gyroscope needs to depolarize the light beams by interposing a depolarizer (7) to avoid the fluctuation of the scale factor or the zero-point. The ground of the necessity of depolarization is now clarified briefly.
A single-mode fiber is incapable of maintaining the polarization due to the perfect rotational symmetry around the axis. In the gyroscope, the light enters the fiber coil (5) after the light has been converted into linear polarization in a certain direction by the polarizer. The polarization of the light is likely to rotate in the single-mode fiber coil by some reasons, for the single-mode fiber has no function of keeping the polarization in a constant direction. When the light beams enter the polarizer in the reverse direction, the beams are not always able to pass the polarizer. If the polarization were rotated at 90 degrees in the coil, the light could not pass the polarizer at all. Therefore a depolarizer is interposed in the fiber path for depolarizing the light beams before returning to the polarizer. The depolarizer enables half of the light beams to pass the polarizer in any case. The interposition of a depolarizer reduces the fluctuation of the scale factor.
The depolarizer has another role. There is an optical path difference between two beams with perpendicular polarizations due to the anisotropic fluctuation of refractive index, although two beams travel in the same single-mode fiber in the same direction. The optical path difference would induce the drift of the zero-point. To suppress the occurrence of the optical path difference between two beams with orthogonal polarizations, the polarizer with a transparent axis is interposed in the fiber path for restricting the polarization in the transparent axis direction. Extinction rate is a parameter representing the power of a polarizer to convert light beams into linear polarization beams. The extinction rate is defined as a quotient of the light power of the polarization vertical to the transparent axis divided by the light power of the polarization parallel with the transparent axis of the polarizer. A polarizer of an extinction ratio of 0 is a perfect polarizer. Extinction ratio has any value between 0 and 1. However a practical polarizer has a definite extinction ratio. Thus an actual polarizer cannot entirely suppress the transit of the light of the polarization vertical to the transparent axis.
The necessary extinction ratio of a polarizer depends upon the state of the polarization of incident beams. If incident beams have various polarizations with high coherency, the incident beams demand an extremely small extinction ratio for a polarizer. However it is hard to fabricate a polarizer with such a small extinction ratio. On the contrary, a depolarizer endeavors to forbid two light beams with orthogonal polarizations from interfering with each other by giving the two beams an optical path difference longer than the coherent length of the light. The depolarizer alleviates the load of the polarizer by suppressing the interference between the two beams with various polarizations. Namely, the depolarizer has the complementary role of suppressing the drift of the zero-point as well as the polarizer has been interposed for suppressing the drift.
Lyot's depolarizer is a well known depolarizer which is produced by gluing two birefringent bulk crystals with the principal (anisotropic) axes inclining at 45 degrees. Instead of Lyot's bulk crystals, a fiber-type depolarizer can be fabricated from two birefringent fibers. FIG. 3 shows a fiber-type depolarizer made by splicing two birefringent fibers (polarization maintaining fiber) with the anisotropic axes twisted to each other at 45 degrees. Since the birefringent principal axes incline reciprocally at 45 degrees, the spliced two anisotropic fibers become a depolarizer.
Now a depolarizer is further explained prior to clarifying the problems of prior art. There is a birefringent material which has a length L, an extraordinary refractive index n.sub.e and an ordinary refractive index n.sub.o. The material yields an optical path difference .DELTA.L between an extraordinary beam and an ordinary beam. Then the lengths of the fibers must be determined so as to maintain the path length longer than the coherent length of the light. EQU .DELTA.L=.vertline.(n.sub.e -n.sub.o)L.vertline.&gt;Lc (1)
Two light wave packets which are distanced by a length longer than the coherent length do not interfere with each other. Therefore the extraordinary beam and the ordinary beam, which have passed the birefringent material satisfying Inequality (1), don't interfere with each other, because two packets will be distanced farther than the coherent length after the transit of the birefringent material. A depolarizer is composed of two birefringent materials satisfying this condition.
In many cases, the ratio of the length is determined to be 1:2. An end of a second birefringent element is glued to an end of a first birefringent element in a posture wherein the anisotropic axis (principal axis) of the second birefringent element is inclined at 45 degrees to the anisotropic axis (principal axis) at the connected ends. When a light beam travels in the first birefringent object, the beam is divided in an arbitrary ratio to an extraordinary beam with a polarization parallel to the anisotropic axis and an ordinary beam with a polarization perpendicular to the anisotropic axis. When the beams pass the interface of two birefringent media, the extraordinary beam and the ordinary beam are respectively divided into precise halves in power. Each of the two wave packets is again divided into an ordinary beam and an extraordinary beam and propagates in the second birefringent medium. Therefore when four wave packets go out of the end of the second medium, the extraordinary beams with the polarization parallel with the anisotropic axis have exactly the same power as the ordinary beams with the polarization vertical to the anisotropic axis. Thus any beam with an arbitrary polarization has the same power, since any polarization can be composed by a linear combination of the ordinary beam and the extraordinary beam with coefficients whose square sum is always equal to 1.
Four different optical paths are generated by the birefringency of two media. An assembly of light waves emitted from a light source at the same time is called a wave packet. The first medium produces two wave packets by its birefringency. The second medium multiplies the wave packet by 2. Thus four wave packets run with separations after the end of the second medium. If the ratio of the two media is set to be 1:2, three distances between any neighboring wave packets are all the same. The common distance is longer than the coherent length Lc. Thus four wave packets no longer interfere with each other. Since any beam with an arbitrary polarization has the same power and any two wave packets of beams do not interfere with each other, the state can safely be called "depolarized". Of course, the non-polarization state is not perfect as a non-polarization state of an incandescent lamp. Nevertheless, the state the depolarizer prepares is quite similar to the perfect non-polarization state. Thus a depolarizer can be built by gluing two birefringent media satisfying Inequality (1) with the anisotropic axes which are twisted with each other at 45 degrees.